Overview

A join operation is the association of objects in one data source with objects that share a common attribute in another data source.

Join operations target data sources whose relationships to each other cannot be followed directly.

join type

In LINQ, Join implements an inner join.
GroupJoin has no direct equivalent in relational database terms.
It implements a superset of inner joins and outer joins.

methods

Method	Description	Query expression
`Join`	Join two sequences based on key-selector functions and extract pairs of values.	join … in … on … equals …
`GroupJoin`	Join two sequences based on key-selector functions and group the resulting matches from each element.	join … in … on … equals … into …

`Join` example

Join two sequences based on a specific value:

var query = from student in students
            join department in departments on student.DepartmentID equals department.ID
            select new { Name = $"{student.FirstName} {student.LastName}", DepartmentName = department.Name };

In method syntax:

var query = students.Join(departments,
    student => student.DepartmentID, department => department.ID,
    (student, department) => new { Name = $"{student.FirstName} {student.LastName}", DepartmentName = department.Name });

`GroupJoin` example

Join two sequences based on a specific value and group the resulting matches for each element:

IEnumerable<IEnumerable<Student>> studentGroups = 
            from department in departments
            join student in students on department.ID equals student.DepartmentID into studentGroup
            select studentGroup;

In method syntax:

IEnumerable<IEnumerable<Student>> studentGroups = 
    departments.GroupJoin(students,department => 
        department.ID, student => 
            student.DepartmentID, (department, studentGroup) => studentGroup);

inner joins

Assuming collections A and B, an inner join results in a set in which each element in A appears one time for every matching element in B.
If an element in A has no match in B, it does not appear in the result. Inner joins are implemented via the Join method. There are four types of such joins:

Single key join
Composite key join
Multiple join
Inner join using grouped join

single key join

Compare elements based on a single property. Match Teacher objects with Department objects whose TeacherId matches that Teacher:

var query = from department in departments
            join teacher in teachers on department.TeacherID equals teacher.ID
            select new
            {
                DepartmentName = department.Name,
                TeacherName = $"{teacher.First} {teacher.Last}"
            };

In method syntax:

var query = teachers.Join(departments, teacher => 
    teacher.ID, department => 
        department.TeacherID, (teacher, department) =>
            new { DepartmentName = department.Name, TeacherName = $"{teacher.First} {teacher.Last}" });